Sports Illustrated is celebrating the 50th anniversary of its annual swimsuit issue this year. Here, we examine all 50 covers to see how "sexy" has evolved since 1964. Or didn't. (Hint: still boobs.)

In the beginning, the cover shot poses had a much more candid feel — especially compared to recent iterations. Almost like casual snapshots. Another immediately noticeable difference is that in 1964, the first covergirl — Babette March — didn't even fill out the top of her white bikini; in 2013, a parka couldn't even contain all of Kate Upton. However, the belted suit on the 1965 cover established a tradition of regularly featuring swimwear that would leave nightmare tan lines. Some things never change.